HR Diagrams

AST 112

Measurements• We can measure:

– Temperature

– Mass

– Spectra

– Size

– Luminosity

– Distance

• We can make measurements on trillions of stars.

A pattern? Seriously?

Given the many properties we can measureand the trillions of stars that we can observe,

is there any hope of fitting it into a simplescheme? Or is it all just random?

Classification Scheme

• These stars at same distance

• Brightest ones are redor bluish white

• Less bright ones yellow white

• Dimmest ones red

Classification Scheme

• Blue is hotter, red is cooler

• So the photo suggests relation between luminosity (brightness) and color (due to temperature)

• Ejnar Hertzsprung and Henry Russell graphed luminosity vs spectral type, found patterns

Size, Temperature, Luminosity

• Keep the following relations in mind:

– For a given temperature, luminosity goes UP as size goes up

– Temperature sets color

– For a given size, luminosity goes UP as temperature goes up

Hertzsprung – Russell (H-R) Diagram

H-R Diagram

• Very important in astronomy

• Luminosity on vertical axis

• Temperature (or spectral type) on horizontal axis

H-R Diagram

• Notice the star-size indicators on the graph

• If it is a given temperature:– If it’s larger, it gives off

more light

• For a given size, if it’s hotter, it gives off more light

H-R Diagram

• Four major groups:

– Main Sequence

– Supergiants

– Giants

– White Dwarfs

H-R Diagram

• Luminosity Class: Where it lives on H-R Diagram– I: Supergiants– III: Giants– V: Main Sequence Stars– VII: White Dwarf Stars

Stellar Classification

• To completely classify a star, specify:

– Spectral Type

– Luminosity Class

Stellar Classification

• The Sun is G2 V– Spectral type is G2 (yellow-white), Luminosity Class is V

(main sequence)

• Betelgeuse is M2 I– Spectral type is M2 (red), Luminosity Class is I (supergiant)

• Sirius is A1 V– Spectral type is A1 (bluish-white), Luminosity Class is V

(main sequence)

The Main Sequence

• Most stars we observe live on the Main Sequence

• Star’s position along Main Sequence depends on its mass

• Remember that we know mass by looking at binary stars

The Main Sequence

• All stars on Main Sequence fusing H into He

• As mass increases:– H fusion increases– Size increases– Temperature goes up– Luminosity goes up

The Main Sequence

• During a star’s life on the Main Sequence, it doesn’t move very far on the HR diagram.

The Main Sequence

• Stars have finite amount of H

• More massive stars have higher fusion rates, shorter lifetimes

• If we know the mass and know the nuclear reactions, we can estimate the lifetime

The Main Sequence

• Largest Main Sequence stars live 10 million years

• Smallest ones live 1 trillion years

• Larger stars therefore more rare– Their lifetimes are 10,000,000 years. The Universe

is 14,700,000,000 years old. Why do these stars exist?

Stellar Mass

• Ultimately, a star’s mass determines -all- of its properties– Temperature– Spectral type– Luminosity– Size– Fusion rate– Lifetime

Giants and Supergiants

• These have exhausted their supply of hydrogen

• At the end of their lives. Can pull some “tricks” before gravity wins and crushes the star inward.

• They fuse heavier elements. – Fusion rate is very high– Star swells from releasing the energy of fusion

Giants and Supergiants

• Red giants and supergiants:

– They are cooler but more luminous

– They must be larger than the Sun

• Betelgeuse is 500x larger than the Sun!

White Dwarfs

• If a giant (not a supergiant!) has a mass similar to the Sun:

– It expels its outer layers

– What’s left is a dense, hot core

• Book page 535: White dwarfs are “remaining embers of former giants”

Relative Sizes

Variable Stars

• Brightnesses of a star may vary with time

• Outer layers may be opaque– Absorb energy, swell out– Becomes more transparent, lets energy out– Contracts– Repeats

Variable Stars

• Steady repetition of brightness variation

• Period ranges from several hours to several years

Variable Stars

• Variable stars tend to live on a particular part of the H-R diagram

– “Instability Strip”

Cepheid Variable Stars

• Tip of the Instability Strip has Cepheid variable stars

• Period of pulsation closely related to their luminosities

• We use these to measure the distances to other galaxies.